The conservative Heritage Foundation released a report Monday arguing that a bipartisan immigration bill in the Senate — which would grant legal residency to the estimated 11 million undocumented workers currently living in the U.S. — would also stick taxpayers with a multi-trillion-dollar bill. The report has come under heavy criticism, and much of it from an improbable source: Conservatives.

The report claims that the bill would result in undocumented workers receiving a total of $9.4 trillion in government benefits over their lifetimes while paying only $3.1 trillion in taxes — a net loss of $6.3 trillion in taxpayer money. 

"Amnesty is unfair to those who come here lawfully and those who are waiting," said Heritage Foundation President Jim DeMint, who recently stepped down as the junior senator from South Carolina. "It will cost the American taxpayer trillions of dollars over the next several decades, and it will make our immigration problems worse."

That finding would seem to be what conservative critics of immigration reform need: An alarming fiscal, rather than social, basis for opposing it. Yet conservatives have largely criticized the report's methodology, and many have dismissed it entirely. According to the Washington Post, supporters of the immigration bill have already begun circulating a roster of Republicans who are critical of the study.

In short, the study claims that the average undocumented worker will live another 50 years, all while using public services like schools and roads, and collecting medical and other government benefits at a high, disproportionate cost to other taxpayers. But Republican critics have shot back that the study rolls up every conceivable cost associated with legalizing undocumented workers without also including the economic boost newly legal residents would bring — a process called "dynamic scoring."

"The Congressional Budget Office has found that fixing our broken immigration system could help our economy grow," House Budget Committee Chairman Paul Ryan (R-Wisc.) told CQ Roll Call Monday in response to the study. "A proper accounting of immigration reform should take into account these dynamic effects." 

Sen. Jeff Flake (R-Ariz.), who helped craft the legislation as a member of the Gang of Eight in the Senate, similarly pushed back against the report's economic argument.

Political advocacy groups have also struck at the report's underpinnings. The Bipartisan Policy Center, home to Republicans like former Mississippi Gov. Haley Barbour and former Secretary of State Condoleezza Rice, slammed the report as "fundamentally flawed," and accused Heritage of "trying to kill this in the crib."

Barbour himself reportedly said, "It's a political document. It's not serious analysis."

Meanwhile, the Post's Jennifer Rubin, in a column titled "Beware of right-wingers peddling snake oil on immigration reform," warned that Heritage's claims were so clearly skewed that "you have to question the intellectual integrity of those parroting these talking points." As she notes, everyone from the Congressional Budget Office to the libertarian Cato Institute thinks the study is flawed. Cato went so far as to issue a lengthy pre-buttal to the report that cited nearly a dozen errors in a similar 2007 study by one of the report's authors, Robert Rector.

"Not only does Mr. Rector not speak for the broad conservative movement; it appears that economists who have worked for the Heritage Foundation also disagree with Mr. Rector's conclusions," says Cato's Alex Nowraseth.